Bacterial leucine-responsive regulatory protein (Lrp) belongs to a class of conserved DNA-binding proteins widely distributed among prokaryotes and has been the focus of many investigations in recent years. The Lrp of Escherichia coli is the best studied member of this class of proteins, which is a relatively abundant nucleoid-associated protein that functions as a global regulator facilitating bacterial adaptation to a variety of environmental stresses and nutrient starvation. More specifically, Lrp appears to play a key role during transition from exponential to stationary phase of growth. In addition, Lrp-like proteins have been shown to play major roles in virulence mechanisms. Upon searching the Mycobacterium tuberculosis genome we found three Lrp homologs encoded by Rv2324, Rv2779 and Rv3291c. Our goal is to study the role of these Lrp-like proteins in the pathogenesis of M. tuberculosis. Our hypothesis is that Lrp-like proteins play important roles in the transition from exponential (replicative) growth to persistent (non-replicative) phase of growth (i.e., dormancy) and in the transition from the persistent phase back to the exponential phase (i.e., reactivation) in this important pathogen. We have used a genetic approach making knockout mutations in the above genes using specialized transduction technology developed in our laboratory. The effect of these mutations on growth, survival and drug tolerance of M. tuberculosis will be studied under different dormancy-inducing in vitro conditions. The role of the Lrp-like proteins of M. tuberculosis in establishing a persistent infection will be studied using the mouse model of infection and their role in reactivation will be studied using the murine models of reactivation. In addition, to determine the potential roles of these Lrp-like proteins in M. tuberculosis biology and to elucidate the virulence mechanisms regulated by them the above mutants and their isogenic parents will be studied using DNA microarrays and two-dimensional gel electrophoresis under relevant conditions. Preliminary results from our laboratory has indicated that the genes for all three Lrp-like proteins are non-essential in M. tuberculosis, as previously reported for E. coli Lrp, making this investigation feasible. In addition, at least one of these mutants is hypervirulent in SCID mice and appears to be defective ir global gene regulation suggesting that it functions as a global regulator in M. tuberculosis.